This document discusses the structure and function of xylem in flowering plants. It aims to explain how water is transported from roots to leaves through vascular bundles, which contain xylem and phloem. Xylem consists of hollow tubes reinforced with lignin that transport water and minerals upwards. Water movement occurs through root pressure, capillary action, and transpiration pull. Transpiration pull is the dominant force driving water transport in tall plants, where water loss from leaves creates suction that pulls water upwards.

4. LESSON OBJECTIVES
 To learn about the structure and functions of the xylem
 To learn the mechanisms by which water is transported
from the roots to the leaves (and other plant parts) in
flowering plants
 To know the definition of transpiration and the factors
that affect the rate of transpiration

5. VASCULAR BUNDLES
 Tissues that are responsible for the transport of
materials in plants
 Present in all parts of a plant including the roots,
stem and leaves
 Consist of xylem and phloem
Xylem: Transport of water and dissolved
mineral salts
Phloem: Transport of food substances

6. VASCULAR BUNDLES IN A
DICOTYLEDONOUS ROOT

7. VASCULAR BUNDLES IN A
DICOTYLEDONOUS STEM

8. VASCULAR BUNDLES IN A
DICOTYLEDONOUS LEAF

9. XYLEM - STRUCTURE
 Long, hollow, dead tubes forming continuous
columns from roots to leaves
 No cytoplasm in the lumen
 Walls are strengthened by deposits of lignin
 Pits present in xylem walls – enable transport of
water between xylem vessels
 Two xylem cell types: tracheids and vessel
elements

10. Tracheid
Vessel element

11. XYLEM - FUNCTIONS
 Conducts water and dissolved mineral salts from
roots to stems and leaves
 Provides mechanical support within the plant

12. MECHANISMS OF WATER TRANSPORT
IN PLANTS
 Root pressure
 Capillary action
 Transpiration pull

13. ROOT PRESSURE
 Force that helps to drive water upward into the xylem
 Mainly generated by osmotic pressure in the cells of the roots,
which is in turn caused by active transport of mineral ions into
the root cells
 Usually occurs when transpiration rate is low—at a low
transpiration rate, the dissolved mineral ions accumulate in
the root xylem and lower the water potential
 Water then moves from the root hair cells into the xylem,
through the cortical cells and the endodermis, by osmosis

15. CAPILLARY ACTION
 A phenomenon where liquid spontaneously rises in a
narrow space such as a thin tube
 Since xylem vessels in plants are very narrow tubes,
capillary action helps in moving water up the vessels
 The forces of cohesion (attractive forces between water
molecules) and adhesion (attractive forces between
water molecules and the xylem walls) help to move
water upward in the xylem
 Plays an important part in upward movement of water in
small plants

16. video

17. TRANSPIRATION
 Definition: The loss of water vapour from the aerial
parts of a plant, especially through the stomata of
the leaves
 Water continuously moves out of the mesophyll
cells to form a thin film of moisture over their
surfaces
 From the wet cell walls, water evaporates into the
intracellular spaces, and from there diffuses
through the stomata to the surrounding air

18. TRANSPIRATION
 As water evaporates from the mesophyll cells, the
cell sap becomes more concentrated
 Water from cells deeper inside the leaf thus move
into the mesophyll cells by osmosis
 These cells in turn remove water from the xylem
vessels. As they do that, the whole column of water
moves up the vessels from the roots to the
leavesTRANSPIRATION PULL

19. TRANSPIRATION PULL
 The suction force due to transpiration which pulls
water up the xylem vessels
 The main force that drives the transport of water
and dissolved mineral salts up tall trees
 The stream of water up the plant/tree is known as
the transpiration stream

20. FACTORS AFFECTING THE RATE OF
TRANSPIRATION
 Light
 Temperature
 Wind
 Humidity
To be investigated in
online activity

21. SUMMARY
 The xylem is responsible for the transport of water
and dissolved mineral salts in flowering plants
 Root pressure, capillary action and transpiration
pull are the three mechanisms of water transport in
flowering plants
 Transpiration pull is the main mechanism of water
transport in tall trees

22. Before we end, let’s view this animation
to remind us of what we have learnt
today 